Question

Two lenses A and B have focal lengths of +20 cm and, −10 cm, respectively.
(a) What is the nature of lens A and lens B?
(b) What is the power of lens A and lens B?
(c) What is the power of combination if lenses A and B are held close together?

Answer 1

(a) The focal length of lens A is +20 cm. The positive sign indicates that lens A is convex, i.e., a converging lens.
     The focal length of lens B is -10 cm. The negative sign indicates that lens B is concave, i.e., a diverging lens.

(b) Focal length of lens A, f_A = +20 cm = +0.20 m  

∴ Power of lens `A, P_A=1/(f_A)=1/0.20=5D` 

Focal length of lens B, f_B = -10 cm = -0.10 m 

∴ P​ower of lens B,  `P_B=1/f_b=1/-0.10=-10D` 

(c) When lenses are combined, the power of the combination is the algebraic sum of the powers of the individual lenses.
∴ Power of combination of lens A and B,p=P_A=P_B 

                                       `  P=5D-10D=-5D.`